Thimbles of contact fingers are conventionally manufactured as follows: the starting material is a tube made of a metal that is a good conductor of electricity and that has good properties of resilience, for example and alloy of copper including chromium. One of the ends of the tube is provided with a ring of cupro-tungsten for use in making the contact tips of the fingers. The ring is made by electron bombardment or by brazing.
Parallel longitudinal slots are formed through the ring and along a portion of the tube so as to constitute contact fingers. At present, the cheapest method of making the slots is to perform milling passes; however, that method is unsuitable for making slots that are narrower than 1.2 mm, whereas thimbles of contact fingers in gas blast circuit breakers require the slots to be only about 0.2 mm wide so that the blast gas can be compressed sufficiently. It might be thought that the fingers could be separated by machining using a wire, however although such a method is suitable for obtaining slot widths of the desired narrowness, it suffers from the severe drawback of being lengthy and expensive. The present procedure is therefore to take a tube having wide slots and then to perform an upsetting operation on a lathe until the slots have been narrowed to the desired width. Unfortunately, this operation suffers from the defect of work hardening the metal, thereby causing it to lose a portion of its strength and resilience.
An object of the invention is to provide a method enabling the end of the tube to be swaged without spoiling the qualities of the metal.